In power plants using coal as fuel, etching occurring due to the collision of coal cinders with inner surfaces of pipes and the like, during combustion gas exhausting processes, may be a factor in seriously affecting a lifespan of pipes or structures. In particular, in portions thereof with which coal cinders collide, corrosion may occur faster than in other portions thereof having widened surface areas, as well as the occurrence of etching therein. Such erosion due to coal cinder collisions may be prevented by improving wear resistance. Wear resistance has physical properties in proportion to strength and may be improved by increasing the strength of steel sheets. As a representative method for increasing the strength of steel sheets, solid-solution hardening may be employed, and as representative solid-solution hardening elements, silicon (Si), phosphorus (P) and the like may be used. However, in general, silicon (Si) has a problem in that red scale may occur with the use thereof, and although phosphorus (P) has relatively high reinforcement effects and is relatively cheap, it has been known that P deteriorates corrosion resistance.
In general, it has been known that in the case of corrosion resistant steel for resistance to complex corrosion from sulfuric acid and hydrochloric acid, a large amount of copper (Cu) is added to steel in order to delay the occurrence of corrosion under an atmosphere of sulfuric acid and hydrochloric acid. Although Cu has a remarkable effect of significantly delaying a corrosion speed based on sulfuric acid as compared to other added elements, when a large amount of Cu is added, cracks and the like may occur at the time of performing hot rolling. In addition, since Cu has a relatively low melting point, when a large amount of Cu is added, Cu is extruded, causing the occurrence of cracks in a corner portion, or the like, of slabs to remain as surface defects therein. When portions having such surface defects are exposed to an environment in which corrosion may occur, corrosion may occur therein faster than in other portions, or at the time of processing thereof, fractures may occur therein faster than in other portions. Accordingly, as in Patent Documents 1 to 3 below, steel to which an appropriate amount of Cu is added and other elements are compositely added has been developed, but as the content of Cu is reduced, corrosion resistance is deteriorated.
Meanwhile, during a re-heating process to allow slabs to be subjected to hot rolling, relatively thick scale is formed on surfaces thereof, and a majority of the scale is removed by a high-pressure water jet before and after rough milling is performed. However, when an amount of scale having a component of Fayalite (Fe2SiO4) is formed, the scale is not completely removed even using a high-pressure water jet, causing the occurrence of red scale after hot rolling is performed and causing stains to remain on surfaces thereof, such that the appearance thereof may be degraded and surfaces may not be uniform. Accordingly, since the formation of corrosion is not uniform in an environment in which corrosion may occur, another defect may occur thereby.
(Patent Document 1) Japanese Patent Laid-Open Publication No. 1997-025536
(Patent Document 2) Japanese Patent Laid-Open Publication No. 1998-110237
(Patent Document 3) Korean Patent Laid-Open Publication No. 2009-0070249